Microporous thermal insulation materials are materials which have a lattice structure in which the average interstitial dimension is less than the mean free path of the molecules of air or other gas in which the material is arranged. This results in a heat flow which is less than that attributable to the molecular heat diffusion of air or other gas in which the material is used. The lattice structure is created within a powder material by using a powder with very fine particles which adhere to each other in a chain-like formation. A suitable powder for providing this structure is finely divided silica in the forms normally referred to as silica aerogel or pyrogenic silica, although other materials are also available. The powder may be strengthened by the addition of a reinforcing fibre such as ceramic fibre and an opacifier may be added to provide infra-red opacification.
The microporous thermal insulation material may be formed into a panel by forming an envelope of a porous material, introducing the microporous material into the envelope and compressing the envelope to form a panel in which the compressed microporous material is bonded to the porous material of the envelope.
The porous material used for the envelope is generally a cloth formed of glass fibre. However, we have found that commercially-available glass fibre cloth is not entirely suitable.